The engine is forced to breathe through two tiny 29.4mm restrictors by homologation rules. .... Since the airflow to the engine is limited, the engine is redlined at a relatively low 8,600 rpm as there is simply not enough breathing to efficiently rev higher. (edit from last seasons M3 GTR description found here): http://www.motoiq.com/magazine_artic...m3-coupes.aspx